Remote XUL Manager - Firefox Addon - Support for Remote XUL was removed on Firefox 4. But there's still a whitelist that enables Remote XUL, which can be edited using this add-on.
How to use:
1. Enter about:remotexul in the address bar.
2. To add a domain, click on the Add button and then enter the domain name of the site you want to add to the list (www.mozilla.org, for example).
3. That's it! You should be able to access the site again without any problems.
4. It's recommended that you bookmark the Remote XUL page so that it's easy to come back to it.